1. Field of the Invention
This invention relates to an apparatus for a continuous production of alumino-silicate zeolites, and more particularly to an apparatus for a continuous down-flow zeolite production.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline alumino-silicates having a definite crystalline structure within which there are a large number of channels. These cavities and channels are precisely uniform in size. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline alumino-silicates. These alumino-silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may often be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given alumino-silicate by suitable selection of the cation. The spaces between the tetrahedra are usually occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic alumino-silicates. These alumino-silicates have come to be designated by letter or other convenient symbols, e.g., zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), zeolite ZSM-35 (U.S. Pat. No. 4,016,245) and zeolite ZSM-38 (U.S. Pat. No. 4,046,859), and zeolite ZSM-23 (U.S. Pat. No. 4,076,842).
Zeolite crystallization is commonly conducted batchwise in large autoclaves, either static or stirred. It frequently requires many hours for completion, and can be, by petroleum industry standards, labor-intensive. Manufacturing processes for commercial zeolites can be classified into two groups, those using homogeneous or heterogenous hydrogels and those based on pre-formed gels, for example, pelletized gels. The hydrogel processes conventionally employ large vats or autoclaves for stepwise mixing, gel aging and final crystallization, and several examples are reviewed in Chapter 9 of ZEOLITE MOLECULAR SIEVES by D. W. Breck, John Wiley and Sons, 1974. Although it is possible to crystallize some of these zeolites in a continuous-stream process (as claimed, for example, in Belgian Pat. No. 869,156, July 20, 1978), stepwise, batch processes were preferred in prior art because continuous crystallization processes had either not been demonstrated to apply to specific zeolite systems or had not been sufficiently developed.
The apparatus of the present invention is specifically adapted for synthesis of a class of zeolites characterized in their preparation by reaction mixtures of lower alkalinity and by a zeolite product of SiO.sub.2 /Al.sub.2 O.sub.3 greater than 12. In addition, these zeolites have a constraint index of between 1 and 12, and they are generally prepared in the presence of a nitrogen (N) or phosphorus (P) containing organic compound. For a further description of that method and zeolites produced thereby on a continuous-stream basis, see copending U.S. Application Ser. No. 47,538, filed June 11, 1979, whose entire contents are incorporated herein by reference.
As will be seen from the above-noted copending U.S. application, the zeolite made by the present inventive apparatus are prepared at OH/SiO.sub.2 mole ratios below 1.0 and often below 0.5. The combination of low OH/SiO.sub.2 and highly siliceous reaction mixtures results in gels which are quite stiff and difficult to mix. It is with these reaction mixtures that a continuous-stream crystallization process affords unique advantages, both in terms of production efficiency and in terms of product quality control. Possible advantages of a continuous-stream process include facile and independent control of nucleation and of growth stages of crystal formation by such techniques as temperature and pH gradients, by staged injection of nutrients such as SiO.sub.2 and Al.sub.2 O.sub.3 source materials and of crystallization modifiers such as N- or P-containing organic compounds, alkali metal salts, acids and bases, and by seeding.
An up flow continuous flow zeolite crystallization apparatus for producing the aforementioned zeolites is disclosed in abandoned U.S. application Ser. No. 047,536, filed June 11, 1979 by L. D. Rollmann and E. W. Valyocsik. Although the Rollmann et al application briefly states that in the apparatus disclosed therein the liquid flow "may be either up or down", the detailed description of the apparatus is limited to the operation thereof with the liquid flowing in the upward direction. In addition, it will be obvious to those skilled in the art that the construction of the Rollmann et al apparatus is peculiarly adapted to the upflow mode of operation thereof and that the down-flow mode could not be carried out with the arrangement and the construction of the various apparatus parts as disclosed by Rollmann et al. For example, the stirrer of the Rollmann et al apparatus has blades disposed angularly about the vertical shaft of the stirrer at an angle of between 30 and 60 degrees from the vertical in the upward direction. The stirrer must perform two functions in the upflow reactor: (1) mixing action and (2) mechanical lifting action of the zeolite crystals against the natural settling tendency of the zeolite product so that the product crystals can be removed from the reactor.
In the upflow reactor the inlet tube for fresh nutrient fluids is in direct contact with the reacting liquids in the reactor at all times. Fresh nutrients entering the reactor must pass through a bed of precipitated zeolite crystals upon entering the reactor. This is a significant advantage in systems where seeding of the crystallization process is important. However, in systems wherein seeding is less important and the product crystals present an obstacle to the efficient flow of reactants into the reactor, it may be a disadvantage because it may result in the buildup of products and eventual plugging of the inlet tube.
Accordingly, it is the primary object of the present invention to provide an improved continuous flow crystallization apparatus operating in a down-flow mode.
It is an additional object of the present invention to provide a continuous down-flow reactor for the production of zeolites which produces zeolites at substantially improved yields and rates of production than conventional batch apparatus.
Additional objects will become apparent to those skilled in the art from the study of this specification and the appended claims.